If your SIB is a flat bottom slat floor type - a flat surface will be best to spread the weight.
I use a bit of 3/4 inch ply with u bolts clamped to the roof bars to carry my little Seago on the roof whilst deflated in it's carry bag.
The roof bars normally have 4 cycle carriers mounted on them so I remove 2 carriers and clamp the padded board on - thus still allowing me to carry 2 bikes and sib on the roof.
My engines and other Sib gubbins go in the boot
The ply is covered with rubberised foam and I have painted it black since the photo was aken.
I used 3 x 800kg ratchet straps to secure the Sib down.
Have only used this method once - it was a 60 mile round trip and I hit 60mph no worries with thd load still intact upon my return:thumbup:
You could mount a bigger / wider board to your roof and strap your sib onto this.